1. Field of the Invention
The present invention relates gasket and seal materials, and especially gasket materials for use in sealing multiple layers of an apparatus together to contain fluid flow therethrough, such as in a plate and frame heat exchange or filter apparatus.
2. Description of Related Art
In order to maximize fluid flow though a filter or heat exchange apparatus, multiple functional plates are often stacked in series. In the case of a filter, the functional plates comprise filter elements; in the case of heat exchange apparatus, the plates comprise thin (e.g. 0.6 to 1.0 mm) thermally conductive material, such as stainless steel. In either instance, a fluid seal must be established and maintained between each of the plates to assure that leakage does not occur around plates.
A plate and frame heat exchanger uses a large number of plates, often ranging from 8-16 to 500 or more plates in a single unit, to provide a mechanism for heat transfer. Generally, each of the plates comprises an essentially rectangular element with upper and lower ports therein. When the plates are stacked with their faces parallel to each other and sealed along their edges, cells are created with fluid flow directed through each of the cells, across the face of the plates, from an upper port to a lower port or visa versa.
Heat exchange is accomplished by stacking many plates in this manner and establishing two distinct fluid paths through the heat exchanger. A first path passes up and down the faces of the plates in every other cell (e.g. passing from left to right in the apparatus through the first, third, fifth, etc. cells); and an counter-current second fluid path passes down and up the plates in the alternating cells (e.g. passing from right to left in the apparatus through the sixth, fourth, second cells).
The most important feature of any heat exchanger is its successful ability to segregate and contain the fluids passing through it. Clearly leakage or, worse, intermixing of fluids in the exchanger compromises or completely destroys its proper function. Moreover, where hazardous chemicals are handled by such a system, any leakage can be disastrous. As a result, particular attention is given to providing effective seals between each of the plates of a heat exchanger.
A typical heat exchanger seal comprises a ring of elastomer (e.g. butyl rubber, neoprene, ethylenepropylene diene monomer (EPDM), etc.) or compressed sheet (e.g. asbestos or synthetic fiber) that is mounted around the periphery of each plate and around appropriate ports to assure proper fluid separation and orientation. These gaskets are generally glued in place on the plate and then the plates are stacked within a frame and torqued down until a tight fit is created between each of the plates and the intermediate gaskets.
Existing plate and frame gaskets have many serious drawbacks. For instance, a sometimes difficult compromise must be struck between a material which provides a tight seal and a material which is adequately durable and chemical/heat resistant for long-term use. Another common problem is that the gasket material is required to conform to compensate for misaligned, bent, scored, or otherwise defective plates.
One of the most serious problems with existing plate and frame gasket materials is that many materials (e.g. asbestos or elastomer glued in place with an epoxy or other adhesive) are extremely difficult to remove and replace during routine maintenance. For example, it is estimated that 1 to 1.25 man-hours are required for the reconditioning of each plate in a medium size heat exchanger employing elastomeric gasket material. For a heat exchanger with over 100 plates, reconditioning of the entire heat exchanger is clearly a massive undertaking. Additionally, materials such as EPDM, neoprene, or butyl rubber can have a relatively short operative life of only 6 to 9 months in harsh environments (e.g. when subjected to excessive chemical or thermal attack).
Complicating both the installation and reconditioning of heat exchanger gaskets is the use of texturing on the plates normally employed in these systems. As is known, by corrugating or otherwise texturing a heat exchanger plate, its surface area is significantly increased, thus improving its ability to transmit heat. Further, plate texturing also makes it quite difficult to remove broken gasket material which can become stuck in the plate's ridges.
Another complicating factor in sealing many plate and frame apparatus is that the plates are often poorly supported within the frames. "Non-optimum" plate and frame apparatus support the plates through use of an ill-fitted connection between the plates and guide rails. As a result, the plates must be carefully torqued down to assure that proper alignment is maintained between the plates.
A better system is offered in so-called "optimum" plate and frame apparatus that use closely matched guide rails to help keep the plates aligned during the installation procedure. Such alignment is considered critical where significant compression must occur during installation. Despite this improvement, care must still be maintained to assure that the plates are evenly mounted.
It is important that the plates are kept in contact with each other during service. Such contact is important for increasing heat transfer along the corrugations, as well as compensating for different pressures between plates and cyclic pumping actions which can lead to flexing of the plates and mechanical fatigue. In this regard, the proper placement and maintenance of the gasket material is crucial. The gasket material must supply enough counterforce to seal between the plates during installation; additionally, the gasket material cannot cold flow or "creep" away from the contacting plates by further thinning and widening during use--which could lead to gaps and leakage.
One material that has superior heat and chemical resistant properties is polytetrafluoroethylene (PTFE)As a gasket, PTFE has exhibited utility as a material for use in harsh chemical environments which normally degrade many conventional metals and polymeric materials. PTFE has a usable temperature range from as high as 260.degree. C. to as low as near -273.degree. C.
However, conventional non-porous PTFE gasket materials which have been compression molded or extruded and then heated to a temperature above 345.degree. C. exhibit poor mechanical properties, such as low tensile strength and low cold flow resistance. This limits the use of such materials in areas requiring a measure of physical strength and resistance to creep.
PTFE may be produced in an expanded porous form as taught in U.S. Pat. No. 3,953,566 issued Apr. 27, 1976, to Gore. Expanded polytetrafluoroethylene (ePTFE) is of a higher strength than conventional PTFE, has the chemical inertness of conventional PTFE, and has an increased temperature range of up to 315.degree. C. in service. An example of a porous expanded PTFE gasket material is available from W. L. Gore & Associates, Inc., of Elkton, Md., under the trademark GORE-TEX.RTM. Joint Sealant.
Porous ePTFE joint sealants have proven to have excellent seals in many applications. Unfortunately, due to the inherent compression characteristics of this material, it generally requires a relatively wide sealing surface and a significant clamping load to provide a tight and stable seal between abutting surfaces (i.e. whereby a wide, thin, fully densified gasket can be created). As a result, ePTFE does not perform well in instances with narrow sealing surfaces or requiring relatively thick gasket materials since under compression creep can occur over time to distort the gasket's proper placement. This is a serious constraint in attempting to use this material in the relatively thick-gasketed but high-compression environment of a plate and frame apparatus.
For some applications the problem of creep has been addressed by providing an expanded PTFE core wrapped by a tape of porous ePTFE. Commercial embodiments of such material are available from W. L. Gore & Associates, Inc., under the designation GORE-TEX.RTM. Valve Stem Packing, and Inertech, Inc., of Monterey Park, Calif., under the designation INERTECH.RTM. Valve Stem Packing. These materials are suitable for use as a compression packing where they are confined within a defined volume. However, when used as a gasket in an unconfined volume under a compressive load, these materials exhibit undesirable creep characteristics (i.e. continuing to thin and widen) over time, making them completely unsuitable for use as gasket material in most plate and frame apparatus.
As is demonstrated by U.S. Pat. No. 5,160,773 issued Nov. 3, 1992, to Sassa, very successful use of a coated expanded PTFE seal can be achieved in low compression applications, such as in a "wiper" seal for moving surfaces with very low clamping forces and low fluid pressures. In that case, the sealing material comprises a PTFE felt encapsulated by a porous PTFE sheet laminated to a melt-processible thermoplastic fluoropolymer. Regretfully, where significant compression forces are applied, deformation and undesirable creep is again experienced.
One suggestion for achieving the chemical resistance of PTFE but limiting the amount of creep of the material is to coat a generally creep-stable material such as synthetic rubber with a coating of PTFE to provide chemical resistance. One example of such a structure is presented in U.S. Pat. No. 4,898,638 issued Feb. 6, 1990, to Lugez. In this patent it is taught that through a disclosed process one or more films of only partially porous PTFE can be adhered to a rubber sheet to provide a gasket material with chemical resistance. While this approach may address some of the problems with existing plate and frame gasket materials, the PTFE film can crack under the stresses of compression, leading to exposure and failure of the core elastomer. Further, it is believed that longer life and better thermal and chemical resistivity are possible if an expanded PTFE material is employed throughout the gasket.
As is disclosed in co-pending U.S. patent application Ser. No. 050,903, filed Apr. 20, 1993, it has been determined that a PTFE sealing material can be produced with limited long-term creep by wrapping a core of elongated or expanded PTFE with a high strength film of expanded PTFE. The high strength film is resistant to deformation and stretching and serves to contain the PTFE core in place within a compressed gasket. This material has proven to be quite effective in sealing plate and frame heat exchangers--providing thermal and chemical protection, long-life and durability, and ease in replacement.
Despite the success of the above described material in sealing plate and frame heat exchangers, it has a number of deficiencies. Perhaps the greatest problem with the high strength film wrapped PTFE material is that it requires extensive compression before becoming properly seated within a plate and frame apparatus. A typical gasket material with a rectangular cross-section generally must be compressed in height down about 3:1 before proper seating and sealing is established.
This seating problem is a very serious concern in an application with many plates since a normal corresponding frame is simply too small to contain all the plates and un-condensed gasket material at one time. As a result, an installer must go through the burdensome and time consuming procedure of installing and compacting the plates and gasket materials in a number of batches. This problem is vastly compounded in non-optimal plate and frame apparatus where a large degree of movement of the plates in the sealing process leaves too much room for plate distortion and gasket shifting.
Accordingly, it is a primary purpose of the present invention to provide a gasket material for a plate and frame apparatus that provides an effective long-term seal under pressure, while being durable, chemical and thermal resistant, non-contaminating, and relatively easy to install.
It is another purpose of the present invention to provide a gasket material for plate and frame apparatus that is readily removed and replaced with minimal labor and expense.
It is still another purpose of the present invention to provide a gasket material for plate and frame apparatus that provides the benefits of expanded PTFE material, while avoiding the problem of creep.
It is yet another purpose of the present invention to provide a gasket material for plate and frame apparatus that can be readily installed without requiring undue torque or plate movement.
It is a further purpose of the present invention to provide a method for making and optimally using a gasket material with the above properties.
These and other purposes of the present invention will become evident from review of the following specification.